a. Field of the Invention
The invention relates to bypass valves for use in wellbores, particularly but not exclusively to bypass valves used during the setting of hydraulic anchor packers.
b. Description of Related Art
The drilling industry often has need to monitor the depth and angular orientation of a tool (such as a whipstock) within a wellbore and to rigidly secure the tool within the wellbore once a required position has been achieved. The depth and orientation of a tool is typically determined through use of a measurement-while-drilling (MWD) tool. However, MWD tools require a flow of wellbore fluid through a string in order to communicate a measured depth and orientation to the surface and the flow rates involved are often sufficiently high to prematurely set the hydraulic anchor packer in use.
To overcome this problem, strings are often provided with a bypass valve located between the MWD tool and the anchor packer. When the depth and orientation of the string is being monitored, wellbore fluid is pumped through the MWD tool via the string bore and then bled to the wellbore annulus so as to prevent the pressure differential across the hydraulic anchor packer rising to the level required for setting. Once the string has been arranged in the desired position, the hydraulic anchor packer is set by increasing of the flow rate of wellbore fluid down the string. The increase in flow rate results in an associated increase in dynamic pressure at the bypass valve. Once this dynamic pressure increases to a predetermined magnitude, the bypass valve is activated and the fluid path between the wellbore annulus and the string bore is closed. The wellbore fluid is thereby directed downhole to the anchor packers where the appropriate setting pressure (typically a 1500-3000 psi differential between the inside and outside of the anchor packer) is then applied.
A conventional bypass valve incorporates a piston which slides within a cylinder in response to dynamic wellbore fluid pressure. The wall of the cylinder is provided with a plurality of holes through which fluid may pass from the string bore to the wellbore annulus. The piston is held by biasing means (such as a spring), a shear pin or a combination thereof so as to permit fluid flow through said holes in the cylinder. However, when the predetermined dynamic pressure is achieved, the biasing means and/or shear pin is overcome and the piston slides within the cylinder so that said holes become sealingly closed.
A problem associated with this type of bypass valve is that no warning is given at the surface of an imminent closing of the bypass valve and, consequently, of a potentially imminent setting of the anchor packer. A bypass valve is disclosed in UK patent application no. 9625547.6 (publication no. GB 2 307 932 A) which incorporates means for controlling the movement of the piston within the cylinder. The disclosed arrangement is such that movement of the piston is initially restricted so that the cylinder holes are only partially closed. The restricted passage to the wellbore annulus thereby created results in increased pressure losses which may be detected at the surface. Nevertheless, the dynamic pressure at the bypass valve has been allowed to rise to the predetermined activating magnitude and remedial action (i.e. a cycling of the bypass valve) must then be taken before full closure of the cylinder holes can be achieved. This remedial action is time consuming and, in certain applications, can be inconvenient and potentially problematic.
Further prior art bypass valves to which the present invention pertains are disclosed in U.S. Pat. Nos. 4,768,598 and 5,443,129. The latter document describes a bypass valve according to the preamble of the appended claims. However, this prior art valve requires a partial closing of the fluid path between the valve interior and exterior which is achieved by movement of the piston.
It is an object of the present invention to provide a bypass valve for use in a wellbore which communicates an imminent closure of the bypass valve to the surface.
The present invention provides a bypass valve for selectively isolating the interior of a downhole assembly from the exterior thereof, the bypass valve comprising: a body incorporating a wall provided with at least one opening extending therethrough; a piston slidably mounted in the body such that a first position of the piston relative to the body establishes a passage from the interior of the body to the exterior of the body via the opening and such that a second position of the piston relative to the body substantially isolates the interior of the body from the exterior of the body; and means for increasing the force exerted on the piston by a given flow of fluid through the bypass valve such that the resultant force on the piston is insufficient to move the piston to the second position; characterised in that the force increasing means increases the force exerted on the piston in response to a predetermined flow of fluid through the bypass valve.
Thus, a bypass valve according to the present invention may be employed in downhole operations in a similar manner to prior art bypass valves. However, if the rate of fluid flow through the bypass valve is increased (either intentionally or unintentionally) so that said predetermined fluid flow is achieved. then said means is activated. As a consequence, the force exerted on the piston by fluid flowing through the bypass valve is increased. Although the resultant force on the piston is not sufficient to move the piston so as to effect closure, the activation of said means generates a reactive force which resists the fluid flow. This resistance can be detected at the surface and thereby provides an indication that the fluid pressure differential across the length of the piston has increased to a predetermined level and that further unchecked increases will result in closure of the bypass valve.
The force increasing means preferably comprises means for restricting the passage of fluid past the piston. Furthermore, the passage of fluid past the piston is preferably provided by a fluid pathway comprising a longitudinal bore extending through the piston. The fluid pathway ideally also comprises at least one aperture in the piston providing fluid communication between the piston bore and a fluid route past the piston being at least partially located exteriorly of the piston. In such an arrangement, the passage restricting means preferably comprises a second piston mounted in said piston bore so as to be slidably moveable between positions in which said at least one aperture is either open, closed or partially closed. It is preferable for the second piston to be biased into a position wherein said at least one aperture is open. Said piston may be biased by mean of a spring. Alternatively, the second piston may be held by means of a shear pin in a position wherein said at least one aperture is open. Preferably, the second piston is moveable into a position wherein said at least one aperture is closed. The second piston is preferably provided with a longitudinal bore extending therethrough.
Preferably, the geometry of the piston is such that the piston, once in said second position, is biased into said second position by means of a static fluid pressure differential across said piston.
A bypass valve according to the present invention thereby has the advantage over the prior art of providing an indication at the surface of an imminent closure of the bypass valve. Once said indication is detected, the bypass valve may be closed, without the need for remedial action, by simply increasing the rate of fluid flow down the associated string.